The present invention relates to gas turbines and, in particular, to an air/fuel premixer for a gas turbine.
Typically, gas turbine engines mix compressed air with fuel for ignition in a combustor to generate combustion gases from which mechanical energy or electrical power are generated. The typical air pollutants produced by gas turbines burning conventional hydrocarbon fuels are nitrogen oxides (NOx), carbon monoxide (CO), and unburned hydrocarbons. The rate of NOx formation correlates to the peak local fuel-air ratio of the mixture fed into the combustion chamber. To reduce the pollutant emissions, fuel and air may be premixed to a uniform, lean mixture prior to combustion.
The fuel used is often natural gas, synthetic gas, oil or some combination of these. Where oil is used, an oil tip is inserted through a center body of a nozzle, such as a dry low NOx (DLN) style nozzle typically used to burn premixed natural gas. The disadvantage of such an arrangement is that the oil, burns as a diffusion flame with relatively high NOx emissions or a diluent such as steam has to be added to keep emissions low. Efforts to inject the oil through the same passages as the gas have therefore been attempted but found to be problematic due to the differing injector hole size requirements of oil versus gas. Also, injecting from the vane pack risks fouling of the oil along the vane.